1. Field of the Invention
This invention relates to a light beam scanning apparatus. This invention particularly relates to a light beam scanning apparatus free from occurrence of image density nonuniformity caused by variations in surface reflectivity of a rotating polygon mirror used for light beam scanning.
2. Description of the Prior Art
Light beam scanning apparatuses wherein a light beam is deflected and scanned by a light deflector in a main scanning direction on a scanning surface, and at the same time the scanning surface is moved with respect to the light beam in a sub-scanning direction approximately normal to the main scanning direction, have heretofore been widely used, for example, in scanning recording apparatuses and scanning read-out apparatuses. In the light beam scanning apparatuses, a rotating polygon mirror is employed in many cases as the light deflector. The rotating polygon mirror is advantageous from the viewpoint of scanning stability over other light deflectors such as a galvanometer mirror.
However, in general, the rotating polygon mirror is disadvantageous in that surface reflectivity differs slightly among the mirror surfaces, and therefore the intensity of the light beam actually scanning on the scanning surface differs among the mirror surfaces even though the same intensity light beam is made to impinge upon the respective mirror surfaces of the rotating polygon mirror. When the intensity of the scanning beam changes among the mirror surfaces of the rotating polygon mirror, density nonuniformity arises with the recorded image in the case of the light beam scanning recording apparatus. Also, in the case of the light beam scanning read-out apparatus, distortion arises with the read-out image signal, and consequently, density nonuniformity arises with the image reproduced by use of the read-out image signal. The variations in the surface reflectivity arise comparatively markedly among the mirror surfaces of the rotating polygon mirror, and often arise slightly even in a single mirror surface. Also, when the surface reflectivity varies locally in a single mirror surface, the same problems as mentioned above arise.